Central American Gyres, Tropical Cyclones, and Heavy Rainfall

Between late summer and mid-autumn broad low-level cyclonic circulations with spatial scales of 1000-2000 km can develop over Central America on time scales of 1-2 days and persist for 3-5 days. These broad cyclonic circulation regions, which hereafter we will call gyres, can absorb westward-moving tropical cyclones (TCs) from the east (e.g., Matthew in September 2010), disgorge cyclonic circulations to the northeast that later develop into TCs (e.g., Nicole in September 2010), interact with remnant southward-moving cold fronts to encourage weak TC development (e.g., TCs Lee and Nate in September and October 2011, respectively), or interact with weak eastern Pacific tropical depressions (TDs) (e.g., TD 12-E in October 2011). A distinguishing feature of a Central American gyre is that it can be associated with exceptionally heavy rainfall and damaging flooding regionally, such as occurred over parts of Central America in conjunction with the landfall of TD 12-E and TC Nate, and remotely, such as occurred along the Atlantic coast in conjunction with TC Lee.

The purpose of this presentation will be to document the structure and evolution of gyre moisture sources. Emphasis will be placed on understanding how upstream and downstream flow evolution over the tropics and midlatitudes across the Pacific and Caribbean, respectively, contribute to gyre formation and moisture concentration over Central America. The results of selected gyre case studies, drawn from the sample of events mentioned above and other opportunistic events, will be presented. We will target gyre-related moisture sources, gyre-TC interactions, and gyre-induced deep tropical moisture surges into higher latitudes. Gyre-induced deep tropical moisture surges into higher latitudes are especially important because they can result in heavy flood-producing rains. For example, a deep tropical moisture surge associated with the “birth” of TC Nicole from a Central American gyre during PREDICT in September 2010 served as a focus for subsequent heavy rains along the U.S. East Coast. Similarly, deep tropical moisture surges associated with the interactions of TCs Nate and Lee with a Central American gyre in September 2011 were instrumental to the subsequent occurrence of flood-producing heavy rains in eastern Mexico and parts of the U.S. East Coast, respectively.